The smaller ‘twin’ moon is believed to have only survived a few million years before it collided with the one we see today, leaving just one.

The theory will be explained by Professor Erik Asphaug, from the University of California at Santa Cruz at a conference about the moon to be held at the Royal Society this September.

He said: "The second moon would have lasted for only a few million years; then it would have collided with the moon to leave the one large body we see today.

“It would have orbited Earth at the same speed and distance and just got slowly sucked in until they hit and then coalesced.”

Prof Asphaug told the Sunday Times he believes the landscape of the moon, which appears to have mountains, are the remains of Earth’s smaller moon when the pair collided. The moon's smaller twin is believed to have been about onethirtieth of the size.